Frequency Spectrum
Long Waves, Medium Waves and Short
Waves
Communication System
B
A
Engineering System
Social System
Genetic System
History and fact of communication
What is a communications system?

Communications Systems: Systems
designed to transmit and receive
information
Info
Source
Comm
System
Info
Sink
Block Diagram
Info
Source
m(t)
message
from
source
n(t)
noise
Transmitter
Channel
Tx
s(t)
transmitted
signal
Receiver
Rx
r(t)
received
~ (t )
signal
m
received
message
to
sink Info
Sink
Telecommunication
Telegraph
 Fixed line telephone
 Cable
 Wired networks
 Internet
 Fiber communications
 Communication bus inside computers to
communicate between CPU and memory

Wireless Comm Evolution:
UMTS (3G)
http://www.3g-generation.com/
http://www.nttdocomo.com/reports/010902_ir_presentation_january.pdf
Communications












Satellite
TV
Cordless phone
Cellular phone
Wireless LAN, WIFI
Wireless MAN, WIMAX
Bluetooth
Ultra Wide Band
Wireless Laser
Microwave
GPS
Ad hoc/Sensor Networks
Comm. Sys. Bock Diagram
Noise
m(t)
Tx
Baseband
Signal
• “Low” Frequencies
• <20 kHz
• Original data rate
s(t)
Channel
r(t)
Bandpass
Signal
Rx
~ (t )
m
Baseband
Signal
• “High” Frequencies
• >300 kHz
• Transmission data rate
Modulation
Formal definitions will be provided later
Demodulation
or
Detection
Aside: Why go to higher
frequencies?
Half-wave dipole antenna
Tx
l/2
c=fl
c = 3E+08 ms-1
Calculate l for
f = 5 kHz
f = 300 kHz
There are also other reasons for going from baseband to bandpass
Another Classification of Signals
(Waveforms)


Deterministic Signals: Can be modeled as a
completely specified function of time
Random or Stochastic Signals: Cannot be
completely specified as a function of time; must
be modeled probabilistically

What type of signals are information bearing?
Noise



Transmit power
 Constrained by device, battery, health issue, etc.
Channel responses to different frequency and different time
 Satellite: almost flat over frequency, change slightly over time
 Cable or line: response very different over frequency, change
slightly over time.
 Fiber: perfect
 Wireless: worst. Multipath reflection causes fluctuation in
frequency response. Doppler shift causes fluctuation over time
Noise and interference
 AWGN: Additive White Gaussian noise
 Interferences: power line, microwave, other users (CDMA
phone)
Shannon Capacity


Shannon Theory
 It establishes that given a noisy channel with
information capacity C and information transmitted at a
rate R, then if R<C, there exists a coding technique
which allows the probability of error at the receiver to be
made arbitrarily small. This means that theoretically, it
is possible to transmit information without error up to a
limit, C.
 The converse is also important. If R>C, the probability of
error at the receiver increases without bound as the rate
is increased. So no useful information can be transmitted
beyond the channel capacity. The theorem does not
address the rare situation in which rate and capacity are
equal.
C  B log 2 (1  SNR) bit / s
Shannon Capacity
How transmissions flow over media
Simplex – only in one direction
 Half-Duplex – Travels in either direction, but not
both directions at the same time
 Full-Duplex – can travel in either direction
simultaneously

Coaxial Cable
•First type of networking
media used
•Available in different
types (RG-6 – Cable TV,
RG58/U – Thin Ethernet,
RG8 – Thick Ethernet
•Largely replaced by
twisted pair for networks
Unshielded Twisted Pair

Advantages
 Inexpensive
 Easy to terminate
 Widely used, tested
 Supports many
network types
 Disadvantages
 Susceptible to interference
 Prone to damage during
installation
 Distance limitations not
understood or followed
Glass Media
• Core of silica, extruded glass or plastic
• Single-mode is 0.06 of a micron in diameter
• Multimode = 0.5 microns
• Cladding can be Kevlar, fibreglass or even steel
• Outer coating made from fire-proof plastic

Advantages
 Can be installed over long
distances
 Provides large amounts of
bandwidth
 Not susceptible to EMI RFI
 Can not be easily tapped (secure)
 Disadvantages
 Most expensive media to
purchase and install
 Rigorous guidelines for
installation
Wireless
Wireless (2)
 Radio
transmits at 10KHz to 1KHz
 Microwaves transmit at 1GHz to 500GHz
 Infrared transmits at 500GHz to 1THz
 Radio transmission may include:




Narrow band
High-powered
Frequency hopping spread spectrum (the hop is
controlled by accurate timing)
Direct-sequence-modulation spread spectrum (uses
multiple frequencies at the same time,
transmitting data in ‘chips’ at high speed)
Connectors
Fibre Optic
RJ45
Token Ring
Thicknet
T-Piece
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
radio
carrier
21
IIT
Bom
Digital modulation
very simple
 low bandwidth requirements
 very susceptible to interference

t
1

0
1
Frequency Shift Keying (FSK):


Sridhar Iyer
Modulation of digital signals known as Shift Keying
1
0
1
 Amplitude Shift Keying (ASK):

needs larger bandwidth
Phase Shift Keying (PSK):
more complex
 robust against interference
t
1
0
1


22
t
Many advanced variants
IIT
Bom
Multiplexing Mechanisms
Multiplexing
k1

Multiplexing in 4 dimensions
space (si)
 time (t)
 frequency (f)
 code (c)

24
k3
k4
k5
k6
c


k2
t
Sridhar Iyer
channels ki
c
t
s1
f
s2
c
Goal: multiple use
of a shared medium
Important: guard spaces needed!
f
t
s3
f
IIT
Bom
Frequency multiplex
Sridhar Iyer
Separation of the whole spectrum into smaller
frequency bands
 A channel gets a certain band of the spectrum for the
whole time
 Advantages:
k1
k2
k3
k4
k5
k6
 no dynamic coordination
c
necessary
 works also for analog signals

Disadvantages:
 waste of bandwidth
if the traffic is
t
distributed unevenly
 inflexible
 guard spaces

25
IIT
Bom
f
Time multiplex
Sridhar Iyer

A channel gets the whole spectrum for a certain
amount of time
Advantages:
 only one carrier in the
medium at any time
 throughput high even
for many users

k1
k2
k3
k4
k5
k6
c
f
Disadvantages:
 precise
synchronization
necessary t

26
IIT
Bom
Time and frequency multiplex
Sridhar Iyer
Combination of both methods
 A channel gets a certain frequency band for a
certain amount of time
 Example: GSM
k1
k2
k3
k4
 Advantages:

better protection against
tapping
 protection against frequency
selective interference
 higher data rates compared to
code multiplex


27
k5
k6
c
f
but: precise coordination
t
required
IIT
Bom
Code multiplex
k2
k3
k4
k5
k6
Sridhar Iyer
Each channel has a unique code
k1
 All channels use the same
spectrum at the same time
 Advantages:

c
bandwidth efficient
 no coordination and synchronization
necessary
 good protection against interference
and tapping


f
Disadvantages:
lower user data rates
 more complex signal regeneration


28
Implemented using spread
spectrum technology
t
IIT
Bom
Signal
Encoding
Techniques
(modulation and encoding)
Analog
data to analog signal
(AM, FM, PM)
Digital
data to analog signal
(ASK, FSK, BPSK, QAM)
Analog
data to digital signal
(PCM, DM)
Digital
data to digital signal
(line codes)
29
Analog-to-analog conversion is the representation of
analog information by an analog signal. One may ask
why we need to modulate an analog signal; it is already
analog. Modulation is needed if the medium is bandpass
in nature or if only a bandpass channel is available to
us.
Topics discussed in this section:
 Amplitude Modulation
 Frequency Modulation
 Phase Modulation
5.30
5-2 ANALOG AND DIGITAL
5.31
Figure 5.15 Types of analog-to-analog modulation
Analog Signals
32
Digital Signals
33
Analog and Digital Transmission
Prof. N. D. Mehta
35

Modulation
The process by which some characteristics of a carrier
wave is varied in accordance with an informationbearing signal.
 Continuous-wave modulation

Amplitude modulation
 Frequency modulation


AM modulation family
Amplitude modulation (AM)
 Double sideband-suppressed carrier (DSB-SC)
 Single sideband (SSB)
 Vestigial sideband (VSB)

36
Amplitude Modulation
A
carrier signal is modulated only in
amplitude value
 The modulating signal is the envelope of the
carrier
 The required bandwidth is 2B, where B is
the bandwidth of the modulating signal
 Since on both sides of the carrier freq. fc, the
spectrum is identical, we can discard one
half, thus requiring a smaller bandwidth for
transmission.
AMPLITUDE MODULATION
38
AMPLITUDE MODULATION
39
Figure 5.16 Amplitude modulation
Note
The total bandwidth required for AM
can be determined
from the bandwidth of the audio
signal: BAM = 2B.
5.42
Figure 5.17 AM band allocation
AMPLITUDE MODULATION
Carrier wave: is a waveform (usually sinusoidal) that is modulated (modified)
with an input signal for the purpose of conveying information. This carrier wave
is usually a much higher frequency than the input signal.
1.

DEFINING AM
A carrier wave whose amplitude is varied in
proportion to the instantaneous amplitude of a
modulating voltage
GENERATING THE AM
nonlinear device: diode or transistor biased in its
nonlinear region
2.
43
DCTC, By Ya Bao
44
3. ANALYSIS OF THE AM WAVE
vc  Vc sin 2f c t
m
m
v  Vc sin 2f c t  Vc cos 2 ( f c  f s )t  Vc cos 2 ( f c  f s )t
2
2
45
46
4. Different Carriers and AM
Carriers are spaced at 20 kHz, beginning at 100kHz.
Each carrier is modulated by a signal with 5kHz
bandwidth. Is there interference from sideband overlap?
47
5. MODULATION INDEX AND SIGNAL POWER
Vm
Vmax  Vmin
m

Vc
Vmax  Vmin
48
Moduiation Index and Power
2
carr
2
V
2
c
(Vc / 2 )
V
Pc 


R
R
2R
PLSB  PUSB
2
Vc
m


2R
4
2
C
2
2
2
PT
m  2(  1)
PC
V
m
m
PT 
(1 
)  Pc (1 
)
2R
2
2
PT
m
 1
Pc
2
49
Current Calculations
2
IT
m
 1
Ic
2
Example
A carrier of 1000 W is modulated with a resulting
modulation index of 0. 8. What is the total power?
What is the carrier power if the total power is 1000 W
and the modulation index is 0.95?
50
6.2 Double Sideband Suppressed Carrier
(DSBSC)
When the carrier is reduced, this is called doublesideband suppressed-carrier AM, or DSB-SC. If the
carrier could somehow be removed or reduced, the
transmitted signal would consist of two informationbearing sidebands, and the total transmitted power
would be information
51
6.3 Single-Sideband (SSB)

suppressing the carrier and one of the sidebands
52
53
6.4 Filtering the SSB LSB or USB

Dual Conversion: up-converting the modulating
frequency twice and selecting the upper or lower
sideband for transmission.
54
AM: Features and Drawbacks:
the AM signal is greatly affected by noise
impossible to determine absolutely the original
signal level
conventional AM is not efficient in the use of
transmitter power
AM is useful where a simple, low-cost
receiver and detector is desired
55
FREQUENCY MODULATION
56
Frequency Modulation. The carrier's
instantaneous frequency deviation from its
unmodulated value varies in proportion to the
instantaneous amplitude of the modulating signal.
eFM  Ac sin( ct  m f sin mt )
Phase Modulation. The carrier's instantaneous
phase deviation from its unmodulated value
varies as a function of the instantaneous
amplitude of the modulating signal;
ePM  Ac sin( ct  m sin mt )
57
FIGURE 4-1 The FM and PM waveforms for sine-wave modulation: (a) carrier
wave; (b) modulation wave; (c) FM wave; (d) PM wave. (Note: The derivative of
the modulating sine wave is the cosine wave shown by the dotted lines. The
PM wave appears to be frequency modulated by the cosine wave.)
58
5.59
Note
The total bandwidth required for FM can be determined from the bandwidth
of the audio signal: BFM = 2(1 + β)B. Where  is usually 4.
5.60
Figure 5.18 Frequency modulation
5.61
Figure 5.19 FM band allocation
Phase Modulation (PM)
5.62
The modulating signal only changes the phase of
the carrier signal.
 The phase change manifests itself as a frequency
change but the instantaneous frequency change is
proportional to the derivative of the amplitude.
 The bandwidth is higher than for AM.

MODULATION INDEX

modulation index for an FM signal
mf 

fm
δ = maximum frequency deviation of the carrier caused
by the amplitude of the modulating signal
fm = frequency of the modulating signal
63
FREQUENCY ANALYSIS OF THE FM WAVE
eFM  Ac J 0 sin ct
 Ac {J1 (m f )[sin( c  m )t  sin( c  m )t ]}
 Ac {J 2 (m f )[sin( c  2m )t  sin( c  2m )t ]}
 Ac {J 3 (m f )[sin( c  3m )t  sin( c  3m )t ]}
 ..., etc

where: eFm = the instantaneous amplitude of the
modulated FM wave
Ac = the peak amplitude of the carrier
Jn = solution to the nth order Bessel function for a
modulation index mf.
mf = FM modulation index, Δf/fm
64
65
Spectral components of a carrier of frequency, fc, frequency modulated by
a sine wave with frequency fm
66
FM signal characters
• The FM wave is comprised of an infinite number of
sideband components
• bandwidth of an FM signal must be wider than that of
an AM signal
• As the modulation index increases from mf = 0, the
spectral energy shifts from the carrier frequency to
an increasing number of significant sidebands.
• Jn(mf) coefficients, decrease in value with increasing
order, n.
• negative Jn(mf) coefficients imply a 1800 phase
inversion.
67
Carrier Frequency Eigenvalues

in some cases the carrier frequency component, JO, and
the various sidebands, Jn go to zero amplitudes at specific
values of m. These values are called eigenvalues.
68
Bandwidth Requirements for FM

The higher the modulation index, the greater the
required system bandwidth
BW  2(n  f m )
where n is the highest number of significant
(least 1%, or -40 dB; (20 log 1/100 ), of the voltage of the unmodulated carrier)
sideband components and fm is the highest
modulation frequency.
Carson's Rule
BW  2(  f m )  2 f m (1  m f )
69
Amplitude versus frequency spectrum for various modulation indices (fm
fixed, & varying): (a) mf = 0.25; (b) mf = 1; (c) mf = 2; (d) mf = 5; (e) mf = 10.
70
Warren Hioki
Telecommunications, Fourth Edition
71
Copyright ©2001 by Prentice-Hall, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 4-6
Commercial FM broadcast band.
72
Commercial FM broadcast band
•
The maximum permissible carrier deviation, δ, is ±75
kHz
•
Modulating frequencies (voice or music) is ranging from
50 Hz to 15 kHz
•
The modulation index can range from as low as 5 for fm
= 15 kHz (75 kHz/15 kHz) to as high as 1500 for fm = 50
Hz (75 kHz/50 Hz).
•
The ±75-kHz carrier deviation results in an FM
bandwidth requirement of 150 kHz for the receiver.
•
A 25-kHz guard band above and below the upper and
lower FM sidebands.
•
Total bandwidth of one channel is 200Hz.
73
Narrowband FM (NBFM)




NBFM uses low modulation index values, with a
much smaller range of modulation index across all
values of the modulating signal.
An NBFM system restricts the modulating signal to
the minimum acceptable value, which is 300 Hz to
3 KHz for intelligible voice.
10 to 15 kHz of spectrum.
Used in police, fire, and Taxi radios, GSM, amateur
radio, etc.
74
POWER IN THE FM WAVE


power of the unmodulated carrier
Vcrms
PT 
R
For a modulated carrier
2
PT  PJ 0  PJ1  PJ 2  PJ 3  ...  PJ n

2
J0
V
R

2
J1
2V
R

2
J2
2V
R

2
J3
2V
R
 ... 
2
Jn
2V
R
75
FM NOISE
 Increased
bandwidth of an FM – to enhance the signalto-noise ratio (SNR). Advantages of FM over AM.
 To
take this advantage, large mf is necessary– high
order sidebands are important – wider bandwidth is
required.
 Phase
Analysis of FM Noise
VN
  sin
Vc
1
where α = the maximum phase deviation of the carrier frequency caused by the
noise
VN = noise voltage
Vc= carrier voltage
76
Phasor addition of noise on an FM signal’s carrier frequency causes a phase
shift, whose maximum value is .
Warren Hioki
Telecommunications, Fourth Edition
77
Copyright ©2001 by Prentice-Hall, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The ratio of carrier voltage to noise voltage,
Vc
Vn
is the SNR (voltage)
Vc
SNR 
VN
  sin
1
1
SNR
α represents the equivalent modulation index
produced by the noise.
 N    fm
SNRFM


N
78
• The effect of noise on an FM carrier signal is
directly proportional to the modulation frequency fm.
• Increasing fm, degrades the


SNR 

 N   fm
Voice, data, and music contain many frequencies,
which are distributed throughout the given
modulation passband. Therefore, the SNR is not
uniform throughout.
To maintain a flat SNR, some techniques are
employed.
79
Frequency Modulation
5.80
The modulating signal changes the freq. fc of the
carrier signal
 The bandwidth for FM is high
 It is approx. 10x the signal frequency

5.81
Figure 5.20 Phase modulation
5.82
Note
The total bandwidth required for PM can be determined from the bandwidth
and maximum amplitude of the modulating signal:
BPM = 2(1 + β)B.
Where  = 2 most often.
Angle Modulation
83
ANGLE MODULATION:
The intelligence of the modulating signal can be
conveyed by varying the frequency or phase of the
carrier signal. When this is the case, we have
angle modulation, which can be subdivided into
two categories: frequency modulation (FM), and
phase modulation (PM).
84